Same problem Brian in-fact a couple of more since we got it into our org for a migration.
ENV:
WNLB / NO - TMG
4 CAS 2010 servers (Stopped in NLB)
2 CAS 2013 (OWA redirected using 403.4 in Error Pages)
2 MBX 2010 in DAG (All Production mailboxes there - Since all the URLs etc are set correctly no issues on clients connecting)
2 MBX 2013 in NEW-DAG (Since DAG requires the same platform we didn't want it to be 2008R2)
Current Issues:
1. OWA Error in IE 10 - Error:Your request can't be completed right now. Please try again later.
---- Fails on IE 10, reset the browser etc (Because of this issue I am scared of migrating the mailboxes)
Works like a charm on chrome, FF, Safari. Sometimes fails on Chrome as well.
This mailbox was migrated from 2010 to 2013 for testing and (all client connectivity works fine)
2. OWA: Gibberish When connected to a mailbox in 2010
issue in the PIC text itself
Sharique 'W@rbuG' Ahmed. 2 b or not 2 b